602 



SCIENCE 



[N. S. Vol. XXXVI. No. 931 



definition, chosen for its brevity. Dewar and 

 Finn' say that they are " those characters 

 which differ with the sex, but are not directly 

 connected with the act of reproduction." Dar- 

 win" defines them very similarly, but then 

 adds :' 



There are, however, many animals in which the 

 sexes resemble each other, both being furnished 

 with the same ornaments, which analogy would 

 lead us to attribute to the agency of sexual selec- 

 tion. ... It is probable that the ornaments com- 

 mon to both sexes were acquired by one sex, gen- 

 erally the male, and then transmitted to the 

 offspring of both sexes.* 



As far as I can understand, however, he is 

 talking merely of structures, not of those com- 

 binations of a structTire with the instinct for 

 displaying that structure which constitute the 

 real secondary sexual characters. 



The grebe is, so far as I know, unique in 

 this — that structures which are only used in 

 courtship {i. e., which must in origin be due 

 to sexual and not to natural selection) are 

 now not only the common property of both 

 sexes, iut are actually used in display, and 

 used in exactly the same way hy hoth sexes. 



The question of nomenclature remains; 

 what are we to call characters like these, that 

 have arisen through sexual selection, but exist 

 equally in both sexes ? The term " secondary 

 sexual," as we have seen, will not do. A word 

 does exist, however, which is perfectly appli- 

 cable, and that is the word epigamic.^ It 

 would indeed be more satisfactory if we were 

 always to use the term epigamic of all char- 

 acters that owed their origin to sexual selec- 

 tion, whether they are found in one or both 

 sexes, and keep the term secondary sexual in 

 the wider sense that it often has now, to in- 



'"The Making of Species," 1909, p. 298. 



= " Descent of Man," 1871, p. 253. 



'IMd., p. 277. 



* There is no necessity to multiply quotations; 

 I will merely refer the reader to some apposite 

 passages, e. g., Poulton, ' ' Essays on Evolution, ' ' 

 1908, pp. 379, 380; Archdall Eeid, "Laws of 

 Heredity," 1910, p. 145; Weismann, in "Darwin 

 and Modern Science," 1909, pp. 43, 48; in "The 

 Evolution Theory," 1904, Vol. I., pp. 232, 233. 



' Poulton, loc. cit. 



elude all characters peculiar to one sex except 

 the primary sexual characters (of gametes and 

 gonads) and the accessory sexual characters 

 (of genital ducts, copulatory apparatus, etc.). 

 This would cover such epigamic characters 

 (the great majority) that are confined to one 

 sex, as well as many other characters, such as 

 the mammse of female mammals or the wing- 

 less condition of various female moths, which 

 are not epigamic, nor accessory in the strict 

 sense of directly helping the union of the 

 gametes, but have been evolved through nat- 

 ural selection to perform some special func- 

 tion of their own. 



J. S. Huxley 

 Balliol College, Oxfoed, 

 July, 1912 



VOLCANIC ACTION IN THE BLACK HILLS OF SOUTH 

 DAKOTA^ 



In recent work on geology of the norther* 

 Black Hills I have found a sheet of obsidian 

 and associated agglomerates which doubtless 

 are the products of surface extrusion. The 

 locality is li miles northwest of Eoubaix or 

 6 miles south by east of Deadwood in the midst 

 of a large area of pre-Cambrian schists. The 

 many large masses of igneous rocks in this 

 region are well known from descriptions by 

 Newton, Crosby, Jaggar and others, but so 

 far as recorded they are of intrusive nature 

 and of early Tertiary age. The White Eiver 

 deposits (Oligocene) in and about the Black 

 Hills contain large amounts of fragments of 

 these igneous rocks, the products of erosioH, 

 and also much volcanic ash of contemporane- 

 ous origin. The source of this ejected ma- 

 terial has always been a problem, and while 

 the effusive rocks near Eoubaix can not be 

 precisely correlated with White Eiver deposits 

 they appear to indicate that there was volcanic 

 action in this region in mid-Tertiary time. 



The obsidian is a sheet about 15 feet thick 

 and of small extent. Its smooth lower surface 

 lies on a one-foot layer of impure volcanic ash 

 which is underlain by a thick mass of ag- 

 glomerate or flow breccia of apparent rhyolite 



^ Published by permission of the Director of the 

 U. S. Geological Survey. 



